External preheating of fresh air in solid material furnaces

ABSTRACT

The invention relates to a method for utilizing the heat in the lower temperature range of up to 1000° C., preferably up to 500° C., for preheating fresh air which is added to a solid material furnace, preferably designed as a grate furnace with sub-stoichiometric combustion and supply of fresh air in several stages, or to a circulating fluidized-bed furnace.

The invention relates to a method of using the heat in the low-temperature range below 1000° C., preferably below 500° C., in the form of preheated air in a solid-fuel firing system.

An approach is known where fresh air can be heated to high temperatures of below 2000° C. in combustion processes, first of all the oxygen contained in the fresh air being used as an oxidant, and additionally the fresh air itself being used to heat up another medium, in particular, to supply the energy of a water-steam circuit through a heat exchanger, the generated steam being used to drive a turbine.

Most combustion plants, in particular, substitute-fuel plants in which domestic refuse or biomass is burned, have steam boilers to generate medium-pressure steam (below 60 bar). The need to restrict the temperature results from the high-temperature corrosion that is increasingly found with the materials used in the case of steam temperatures above 370° C. to 400° C. These corrosion phenomena can have the result that steam superheaters must be replaced after only a brief operational life of 3 to 12 months. Problems of corrosion and slagging occurring in refuse-burning plants are well-known, as the result of which the temperature in the combustion chamber must be restricted.

Also known in the prior art are biothermal, solar-thermal, and nuclear power plants for generating electricity, that, just like refuse-burning plants and plants that use the waste heat from exothermic chemical processes, are restricted in the maximum temperature achievable in order to convert heat into electricity at a high level of efficiency according to Carnot's theorem.

There is an advantage in combining processes with a limited temperature of the released heat and the combustion process having unlimited, or only slightly limited, fuels, for example coal, oil, gas, or low-load biomasses. The energy and equipment-related cost is nearly independent of the temperature range of the air heating in the case of the combustion process using these types of fuel. Using air, for example, natural gas can be heated to 150° C. so as to initiate protective drying processes, or heated to temperatures of 1600° C. to 2000° C. when the preheated air is further heated.

Due to the increased steam pressure as the result of developments in power-plant engineering and the related possibility of feed-water preheating at increasingly higher temperatures, the flue gas can generally be cooled down to just above the feed water temperature. Limits are no longer placed on feed water preheating relative to the properties of the water due to the use of supercritical steam. In terms of energy, regenerative fresh-air preheating is required and standard practice; flue gases are thus cooled down further and flue gas losses are minimized. The recycled flue-gas heat directly displaces fuel heat and thus constitutes one of the most effective measures for increasing the efficiency of power plants.

In terms of firing technology, fresh-air preheating runs up against certain limits, especially with high-calorific-value solid fuels, since with fresh-air preheating the adiabatic firing temperature also increases simultaneously, with the result that problems are also created in terms of slagging, corrosion, and the material of the refractory linings.

Previously, high-level external fresh-air preheating has been implemented only with furnace systems using oil and gas as fuel. Fuel costs are high, however, and only tolerable for a limited time, and for this reason alternative solutions must be sought. Solar-thermal power plants, for example, are being used to preheat air for gas-turbine power plants. In order for solar-thermal power plants to receive a subsidy, maximum fossil-fuel co-firing rates are specified that rule out a combination of gas-turbine processes with solar-thermal power plants. Solid-material firing systems comprising even preferably regenerative biomass that use external fresh-air preheating are is unknown previously and are the subject matter of this invention.

The prerequisite for these firing systems is flexible temperature control in the combustion chamber that allows the maximum combustion chamber temperature to be limited; to this end, what is proposed is a firing system with circulating fluidized bed and intentional substoichiometric combustion, with subsequent afterburning of the generated low-calorific gas with staged air in a grate furnace.

In principle, solid-material firing systems are known in which the combustion material is first combusted substoichiometrically, and the low-calorific gas or flue gas generated in the first step is combusted completely only in the last stage. Air is added in stages to do this. This firing technology for solid fuels that is implemented practically, for example in the form of a grate-firing systems, pulverized-fuel firing systems, or firing by spreader stoker firing, allows the real temperatures in the furnace chamber to be reduced, thereby enabling a higher-level of fresh-air preheating to be achieved. Known examples include the straw firing in the power plant in Avedøre, Denmark, and the substitute fuel power plant in Södertälje, Sweden with multistage addition of air.

The object of this invention is to provide a method comprising an optimized firing technology for priority solid fuels having the capability of maximum internal and external fresh-air preheating.

This object is achieved by the method according to claim 1. Developments of the invention are described in the dependent claims.

The basic idea of this invention consists in combining fresh-air preheating with a grate-firing system comprising substoichiometric combustion and multiple air staging, or with circulating fluidized-bed firing. Preferably, what is used for firing are solid fuels such as biomass, industrial or municipal wastes, coal, or mixtures of these materials.

Geothermal heat, solar-thermal heat, or heat from refuse incineration and waste heat from exothermic processes are preferably used to effect preheating of the fresh air. One example of waste heat that can be considered is unused district heat during warm-weather periods, for example during the summer. By using this type of district heat, for example, preheating the air to 80° C. to 150° C. can be achieved that, based on the preheating potential, enables energy to be exploited that otherwise would remain unused. In addition, energy can be introduced from solar collectors into an efficient combustion process of a modern biomass power plant, even at northern latitudes, which energy is usable there at an efficiency of more than 35% for generating electric power. The waste heat in combustion processes that can be obtained from the flue gas can be used effectively for fresh-air preheating, an approach that is more effective than using this energy for condensate heating in the water-steam circuit. In grate-type combustion systems, preheated fresh air is added in staged fashion, where the combustion process is effected for an extended period of time substoichiometrically, while the combustible material is only burned completely during the last stages. For example, a mean is temperature of approximately 900° C. can be maintained in a grate-type combustion system by adding additional air as secondary, tertiary, quarternary, and quinternary air, which action should in idealized terms obtain a temperature between 850° C. and 950° C.

Circulating fluidized-bed combustion (ZWS) represents the best available combustion technology for fresh-air preheating. Here cooling is effected by the circulating ash through a fluidized-bed cooler, thereby enabling up to 80% of the rated thermal input to be exploited by cooling ash at an optimum temperature level of 500° C. to 750° C. Fresh-air preheating of up to a maximum of 750° C. is possible for primary air, and up to 1000° C. for secondary air, with circulating fluidized-bed combustion. Solar towers, for example together with fields of heliostats for concentrating radiation and receivers are known from the area of solar thermal energy, where these either directly preheat air to temperatures of 700° C. to 1000° C., or also use a thermal oil circuit with output temperatures of almost 400° C., or use the current state of the art with molten salt to achieve an output temperature of 565° C. so as to enable implementation of external fresh-air preheating.

The following description gives additional advantages or alternative embodiments based on the drawing. Herein:

FIGS. 1-3 are schematic diagrams depicting facilities for implementing the method according to the invention.

FIG. 1 illustrates a combination of a solar-thermal plant and with biomass-fired circulating fluidized-bed combustion ZWS. Radiation is reflected in a solar field 1.2 to a solar tower 1.1 that preheats the air to 300° C. to 700° C., the air being then delivered as preheated fresh air to a circulating fluidized-bed combustion facility 1.3. The heat thus generated is transferred through a fluidized-bed cooler and a waste-heat superheater to saturated steam, after which the superheated steam is passed to a turbine 1.13 to generate power. The condensate returns through a preheater to the feed water pump. The flue gas is cooled in an air preheater 1.7, the flue gas temperature being reduced from 350° C. down to values around 130° C. so as to allow for flue gas cleaning and minimize flue gas losses. The fresh air delivered through the air preheater 1.7 to the solar tower is preheated to approximately 300° C.

The residual heat contained in the flue gas leaving the fabric filter is used to dry the biomass in another air preheater, this biomass being subsequently delivered as fuel to the circulating fluidized-bed combustion facility ZWS.

In the case of the technology shown in FIG. 1, up to 80% of the rated thermal input is exploited through ash cooling at an optimum temperature of 500° C. to 750° C. Fresh-air preheating is possible up to a maximum of 700° C. for primary air and up to a maximum of 1000° C. for secondary air. The advantage of the facility shown in FIG. 1 is the fact that due to the external fresh-air preheating the biomass requirement can be minimized for external superheating of steam.

FIG. 2 illustrates the combination of the above-mentioned facility with solar seawater desalinization (MED). The fresh water is produced is sufficient for irrigating the biomass production area, thereby enabling the biomass used for reheating to be generated locally on site. In addition, parabolic troughs 2.2 are provided that are filled with thermal oil that is heated to approximately 390° C., and used in the customary way to generate steam and to partially superheat the steam. The last superheating stage is provided by the combustion facility that in turn is operated with external solar fresh-air preheating. The live steam fed to the turbines is at a temperature of 540° C. at a pressure of 150 bar.

In the facility shown in FIG. 3, the fresh air is preheated by the flue gas in a heat exchanger to around 165° C. The preheated air is further heated to 350° C. by hot thermal oil from a parabolic trough solar field. A grate-firing system with multiple air staging functions as the combustion furnace, where preheated fresh air is delivered in each added-air supply. Here biomass serves as the fuel.

Another example of an application is the use of waste heat from a small-scale refuse incineration facility to preheat the secondary air of a coal-fired power plant after fresh-air preheating within the power plant. The refuse incineration is effected as stationary fluidized-bed combustion for mechanically dewatered sewage sludge that requires a high level of internal fresh-air preheating. Almost all of the combustion heat can be transferred by thermal oil or molten salt at temperatures of 300° C.-500° C., and used to effect fresh-air preheating for the coal-fired power plant. The gross efficiency of refuse incineration approximately matches that of the coal-fired power plant.

Reference List 1.1 Solar tower 1.2 Solar Field 1.3 ZWS 1.4 Biomass (30%-100%) 1.5 Superheater, eco boiler 1.6 Superheater, intermediate superheater 1.7 Air preheater (Luvo) 1.8 Fabric filter 1.9 Residual-heat heat exchanger 1.10 Biomass dryer 1.11 Heat/cold supply 1.12 High-pressure turbine (HD) 1.13 Low pressure turbine (ND) 1.14 Superheater (UH) 1.15 Boiler (VD) 1.16 Eco 2.1 Solar tower 2.2 Parabolic troughs 2.3 ZWS 2.4 Biomass (30--100%) 2.5 Superheater, eco boiler 2.6 Superheater, intermediate superheater 2.7 Air preheater (Luvo) 2.8 Fabric filter 2.9 Residual-heat heat exchanger 2.10 Biomass dryer 2.11 Multieffect Desalination (MED) 2.12 Sea water 2.13 Fresh water 2.14 Vacuum 2.15 Solar outlet 2.16 Thermal-oil expansion tank 2.17 Superheater (UH) 2.18 Boiler (VD) 2.19 Eco 2.20 High-pressure Turbine (HD) 2.21 Turbine 3.1 Emergency supply 3.2 Delivery 3.3 Dock 3.4 Metal detector 3.5 Conveyor 3.6 Sieve- floor bunker 3.7 Conveyor auger 3.8 Trough chain conveyor 3.9 Boiler 3.10 Wet deslagger 3.11 Slag 3.12 Multicyclone 3.13 CA(OH)₂ (Option) 3.14 Fabric filter 3.15 Silo, airborne dust 3.16 Thermal oil WT 3.17 Residual heat user 3.18 Heat exchanger (Residual VW) 3.19 Air preheater (Luvo) 3.20 Parabolic trough 3.21 Emergency cooler 3.22 heat exchanger (KoVoWa) 3.23 Pressurized water (5 bar) 3.24 Heat exchanger 3.25 Turbine 3.26 Turbine 3.27 Luko (summer use) 3.28 District heat (17.5 MW) 3.29 Recycle 3.30 Preheater 3.31 Supply water 3.32 HD preheater 

1. A method of using heat in the low-temperature range below 1000° C. to preheat fresh air that is fed to a solid-material firing system, implemented as a grate firing system with substoichiometric combustion and the addition of fresh air in multiple stages, or that is added to a circulating fluidized-bed combustion system.
 2. The method according to claim 1, wherein the heat introduced into the combustion process is used together with fuel-based energy to superheat steam of a third process or of the same process of the waste heat source.
 3. The method according to claim 1 wherein biomass, industrial or municipal waste, coal, or mixtures of the above-mentioned materials are combusted.
 4. The method according to claim 1 wherein fresh air preheated in a maximum of five stages is added to the grate firing system.
 5. The method according to claim 1 wherein geothermal heat, solar-thermal heat, heat from refuse incineration, nuclear energy, or waste-heat energy from exothermic processes is used to preheat the fresh air.
 6. The method according to claim 1 wherein the air is preheated to 350° C.-1000° C.
 7. The method according to claim 1 wherein thermal oil or molten salt is used as the heat transfer system to effect subsequent fresh-air preheating. 